Taxonomy & naming
Charax unimaculatus was described by Carlos Alberto Santos de Lucena in 1989 in a paper that added three new species to the genus Charax Scopoli, 1777, from northern Brazil. The original description appeared in the Revue française d'Aquariologie Herpétologie (15(4): 97–104), with type material deposited at the Museu de Zoologia da Universidade de São Paulo (MZUSP) as catalogued by Oyakawa (1998).
The genus Charax belongs to the subfamily Characinae within the large family Characidae and comprises mostly small to medium-sized, laterally compressed, deep-bodied predators distributed across South American river systems. Lucena's 1987 comprehensive revision of the genus provided the taxonomic framework within which the 1989 paper placed C. unimaculatus. The Catalog of Fishes (Eschmeyer, Fricke & van der Laan) treats Charax unimaculatus Lucena, 1989 as the valid name.
Morphology
Charax unimaculatus reaches a maximum recorded length of 3.5 in standard length (SL). Like its congeners it has the strongly laterally compressed, deep-bodied profile characteristic of Charax, with a distinctly humped dorsal outline and a sharply angled ventral contour. The jaws bear strongly curved, fang-like teeth adapted for seizing and holding small prey or removing scales.
The specific epithet unimaculatus — 'one-spotted' — refers to a diagnostic dark blotch on the body, a feature that distinguishes this species from congeners in the original morphological diagnosis. Coloration in life is not well documented in the available literature, but Charax species are generally silvery to pale straw, with the dark spot providing the most readily visible field mark. Fin counts and precise meristic data were established in the 1989 description and its revision context.
Habitat
Charax unimaculatus is distributed in the middle Amazon River basin of Brazil, based on FishBase records. The original description records specimens from northern Brazil in the Amazonas region. The species is classified as benthopelagic, inhabiting the water column in tropical freshwater environments.
Beyond broad basin-level distributional data, detailed habitat preferences — substrate type, vegetation associations, microhabitat depth, or seasonality — have not been reported in the publicly available literature consulted for this account. Like other middle-Amazon Charax, it is presumed to occupy clear-water or mixed-water streams and river margins rather than the open main channel, but confirmed habitat data remain sparse.
Feeding
Members of the genus Charax are broadly characterised as predators of small invertebrates and small fish, and several congeners are documented scale-eaters (lepidophages) in the wild. FishBase assigns C. unimaculatus a trophic level of 3.6 (±0.6 se), consistent with a diet centred on small animal prey — invertebrates, small fish, or both — rather than plant material.
No species-specific diet studies for C. unimaculatus appear in the available literature. Hobbyists keeping Charax species in general report that these fish actively pursue small live prey and can be aggressive toward smaller tankmates. In the absence of direct dietary observations, the trophic estimate based on relatives provides the best available inference.
Mating
No published observations on the reproductive behaviour of Charax unimaculatus in the wild or in captivity are available in the sources consulted. Charax species are characids and are assumed to follow the egg-scattering reproductive strategy typical of the broader family: open spawning in the water column or among vegetation, with adhesive or semi-adhesive eggs and no parental care by either parent.
Within Characinae, males generally do not provide any guarding behaviour, and spawning events are likely brief and opportunistic. Specific signals or courtship behaviour for this species remain undescribed.
Breeding
Captive breeding of Charax unimaculatus has not been reported in the literature available for this account, and the species is seldom encountered in the aquarium hobby. By analogy with other small characids, breeding would involve egg-scattering among fine-leaved plants or over a substrate in warmer, soft-to-moderate water, with incubation lasting several days and no parental care. Eggs and fry would need to be separated from adults to prevent predation.
Given the genus's predatory tendencies, rearing fry alongside adults of this or any other small fish would be inadvisable. Practical breeding data specific to C. unimaculatus remain unavailable.
In the aquarium
Charax unimaculatus is not a standard aquarium fish and is rarely if ever offered in the hobby trade. Any specimens that do appear would require the same broad conditions inferred for middle-Amazon Charax: tropical temperatures (approximately 75–82 °F), soft to moderately hard water, adequate filtration, and a tank with open swimming space and low-level cover. The predatory, deep-bodied body form makes them unsuitable companions for smaller fish.
FishBase rates the species' vulnerability at 10 out of 100, indicating low intrinsic sensitivity to fishing pressure at its documented size. Data on captive behaviour, feeding response to prepared foods, and long-term husbandry remain sparse in the hobbyist literature; those interested in Charax species more generally will find more detailed notes on congeners such as Charax gibbosus and related forms.
Conservation
Charax unimaculatus has not been assessed by the IUCN Red List; it remains in the 'Not Evaluated' category. This is typical for many Amazonian small-fish species described in the late twentieth century that have limited representation in museum collections and have not been the subject of population surveys.
The middle Amazon basin, while subject to ongoing deforestation pressure and habitat modification, still supports extensive areas of relatively intact freshwater habitat. Without range-size estimates, population trend data, or threat assessments specific to this species, no conservation status determination can be made here. Increased systematic survey work across the Amazonian ichthyofauna — much of which remains underdescribed — would provide the baseline data needed to assess species like C. unimaculatus.